What can photophobia tell us about dry eye?

Neuropathic ocular surface pain: Emerging drug targets and therapeutic implications

INTRODUCTION

Dysfunction at various levels of the somatosensory system can lead to ocular surface pain with a neuropathic component. Compared to nociceptive pain (due to noxious stimuli at the ocular surface), neuropathic pain tends to be chronic and refractory to therapies, making it an important source of morbidity in the population. An understanding of the options available for neuropathic ocular surface pain, including new and emerging therapies, is thus an important topic.

AREAS COVERED

This review will examine studies focusing on ocular surface pain, emphasizing those examining patients with a neuropathic component. Attention will be placed toward recent (after 2017) studies that have examined new and emerging therapies for neuropathic ocular surface pain.

EXPERT OPINION

Several therapies have been studied thus far, and continued research is needed to identify which individuals would benefit from specific therapies. Gaps in our understanding exist, especially with availability of in-clinic diagnostics for neuropathic pain. A focus on improving diagnostic capabilities and researching gene-modulating therapies could help us to provide more specific mechanism-based therapies for patients. In the meantime, continuing to uncover new modalities and examining which are likely to work depending on pain phenotype remains an important short-term goal.

Is the Sex Difference a Clue to the Pathomechanism of Dry Eye Disease? Watch out for the NGF-TrkA-Piezo2 Signaling Axis and the Piezo2 Channelopathy

Dry eye disease (DED) is a multifactorial disorder with recognized pathology, but not entirely known pathomechanism. It is suggested to represent a continuum with neuropathic corneal pain with the paradox that DED is a pain-free disease in most cases, although it is regarded as a pain condition. The current paper puts into perspective that one gateway from physiology to pathophysiology could be a Piezo2 channelopathy, opening the pathway to a potentially quad-phasic non-contact injury mechanism on a multifactorial basis and with a heterogeneous clinical picture. The primary non-contact injury phase could be the pain-free microinjury of the Piezo2 ion channel at the corneal somatosensory nerve terminal. The secondary non-contact injury phase involves harsher corneal tissue damage with C-fiber contribution due to the lost or inadequate intimate cross-talk between somatosensory Piezo2 and peripheral Piezo1. The third injury phase of this non-contact injury is the neuronal sensitization process with underlying repeated re-injury of the Piezo2, leading to the proposed chronic channelopathy. Notably, sensitization may evolve in certain cases in the absence of the second injury phase. Finally, the quadric injury phase is the lingering low-grade neuroinflammation associated with aging, called inflammaging. This quadric phase could clinically initiate or augment DED, explaining why increasing age is a risk factor. We highlight the potential role of the NGF-TrkA axis as a signaling mechanism that could further promote the microinjury of the corneal Piezo2 in a stress-derived hyperexcited state. The NGF-TrkA-Piezo2 axis might explain why female sex represents a risk factor for DED.

Opioids and Ocular Surface Pathology; A Literature Review of New Treatments Horizons

This review discusses the role of opioids in the corneal surface and the different pathways and therapeutic methods of management. A literature review was performed using PubMed database. For the database search, the main searching words “opioid” and “topical opioid treatment” were used with the descriptors “cornea”, “ocular surface”, “neuropathic corneal pain”, “corneal sensitivity” and “naltrexone”; original scientific articles and reviews were included to achieve the purpose of the review. The endogenous opioid system has relevant functions in the organism, and in daily use, opioids are used as painkillers. However, these drugs may be employed for other indications as opioid pathways have a wide spectrum. The corneal surface for topical treatment is easily accessible, hence sparing the side effects of systemic opioids. Instillation of opioid antagonist substances, such as naltrexone, increases corneal healing rates and stimulates the division of corneal epithelium cells without deleterious effects. The natural modulation of endogenous opioids controls different forms of pain, including inflammatory and neuropathic pain, both in the ocular surface and in the central nervous system. There are diverse methods in controlling pain using opioids, especially in refractory forms. This review attempts to collect the literature about corneal surface and opioid pathways to provide an overview image and a possible direction of the news treatments.

Photophobia: shared pathophysiology underlying dry eye disease, migraine and traumatic brain injury leading to central neuroplasticity of the trigeminothalamic pathway

BACKGROUND

Photophobia is a potentially debilitating symptom often found in dry eye disease (DE), migraine and traumatic brain injury (TBI).

METHODS

We conducted a review of the literature via a PubMed search of English language articles with a focus on how photophobia may relate to a shared pathophysiology across DE, migraine and TBI.

RESULTS

DE, migraine and TBI are common conditions in the general population, are often comorbid, and share photophobia as a symptom. Across the three conditions, neural dysregulation of peripheral and central nervous system components is implicated in photophobia in various animal models and in humans. Enhanced activity of the neuropeptide calcitonin gene-related peptide (CGRP) is closely linked to photophobia. Current therapies for photophobia include glasses which shield the eyes from specific wavelengths, botulinum toxin, and inhibition of CGRP and its receptor. Many individuals have persistent photophobia despite the use of these therapies, and thus, development of new therapies is needed.

CONCLUSIONS

The presence of photophobia in DE, migraine and TBI suggests shared trigeminothalamic pathophysiologic mechanisms, as explained by central neuroplasticity and hypersensitivity mediated by neuropeptide CGRP. Treatment strategies which target neural pathways (ie, oral neuromodulators, transcutaneous nerve stimulation) should be considered in patients with persistent photophobia, specifically in individuals with DE whose symptoms are not controlled with traditional therapies.

Exploring the Link Between Dry Eye and Migraine: From Eye to Brain

Dry eye and migraine are common diseases with large societal and economic burdens that have recently been associated in the literature. This review outlines the link between dry eye and migraine, which may have implications for reducing their respective burdens. We highlight possible shared pathophysiology, including peripheral and central sensitization, as the potential link between dry eye and migraine. Finally, therapies targeting similar pathophysiological mechanisms between dry eye and migraine are discussed.

Low-dose naltrexone is effective and well-tolerated for modulating symptoms in patients with neuropathic corneal pain

PURPOSE

Neuropathic corneal pain (NCP) is caused by damage or disease of the somatosensory nervous system that innervates the cornea and presents with symptoms of pain or persistent unpleasant sensations, such as burning, dryness, or light sensitivity. This retrospective study aims to assess the efficacy and tolerability of low-dose naltrexone (LDN) in refractory NCP patients.

METHODS

Fifty-nine NCP patients with a centralized component treated with oral LDN 4.5  mg at bedtime for at least four weeks were identified. Thirty out of 59 patients who had a baseline pain score ≥4 on the visual analogue scale had completed the ocular pain assessment survey (OPAS) and presented persistent pain, despite instillation of topical anesthetic drops, were included. Changes in pain scores, comorbidities, side effects, among others, were analyzed. Change in ocular pain scores (scale 0-10) and quality of life (QoL) scores (scale 0-100%) were the main endpoints.

RESULTS

Mean age (years ± SD) was 45.60 ± 19.30 with a white (80.00%) female (73.33%) predominance. Duration of LDN use was 14.87 ± 11.25 months, and the duration of NCP before treatment was 17.53 ± 17.29 months. Eight patients used LDN as a monotherapy, whereas the remaining used it as an adjunct therapy. LDN resulted in a 49.22% decrease in mean pain score from 6.13 ± 1.93 to 3.23 ± 2.60 (p < 0.001). Mean QoL scores by the OPAS were 5.84 ± 2.57 at the first visit and improved to 3.77 ± 2.91 at the last visit (p = 0.023). Common side effects were vivid dreams, headaches, and stomachache.

CONCLUSION

LDN was effective and well-tolerated for NCP treatment.

Longitudinal changes in dry eye symptoms and signs following lifitegrast therapy and relationship to tear osmolarity

Background

This study measured longitudinal changes in dry eye disease (DED) symptoms and signs following lifitegrast therapy and assessed their relationship to tear osmolarity to test the hypothesis that a decline in tear osmolarity is a reliable leading indicator of subsequent improvement in DED symptoms and signs after initiating lifitegrast treatment.

Methods

This phase IV, prospective, single-arm, open-label, 12-week study enrolled subjects aged ≥18 years with eye dryness score ≥40 (0–100 VAS) and tear osmolarity ≥308 mOsm/L. Subjects were prescribed lifitegrast ophthalmic solution 5%, twice daily in each eye. DED symptoms were assessed via VAS at baseline and 2, 6, and 12 weeks. Signs included tear osmolarity, meibomian gland dysfunction, tear breakup time, and fluorescein corneal staining. In post-hoc analysis, subjects with ≥5 mOsm/L decrease in osmolarity over 12 weeks were Responders.

Results

Of 26 subjects in the intent-to-treat population, 23 were female; mean age was 67.4 years. Baseline mean±SD eye dryness was 68.7±16.5 and tear osmolarity was 317.8±8.5 mOsm/L. All seven symptoms (dryness, burning, foreign body sensation, pain, photophobia, itching, blurred vision) declined significantly (P<0.01) from baseline to 6 and 12 weeks. Signs did not change significantly. For 13 Responders, tear osmolarity decreased from baseline to 12 weeks (319.2±8.5 to 300.6±12.3 mOsm/L, P<0.001) and corneal staining trended toward improvement (1.1±0.9 to 0.6±0.7, P=0.136). Among Nonresponders, osmolarity increased from 316.4+8.7 to 329.6+13.9 (P<0.01) and corneal staining showed no change (1.3±0.8 to 1.0±0.7 at 12 weeks, P=0.293).

Conclusions

Lifitegrast reduced DED symptoms among subjects with moderate-to-severe disease (severity defined by VAS for eye dryness). Potential reasons that may underlie the dichotomous effect of drug treatment on tear osmolarity are discussed.

Automated instrument designed to determine visual photosensitivity thresholds

The Ocular Photosensitivity Analyzer (OPA), a new automated instrument to quantify the visual photosensitivity thresholds (VPT) in healthy and light sensitive subjects, is described. The OPA generates light stimuli of varying intensities utilizing unequal ascending and descending steps to yield the VPT. The performance of the OPA was evaluated in healthy subjects, as well as light sensitive subjects with achromatopsia or traumatic brain injury (TBI). VPT in healthy, achromatopsia, and TBI subjects were 3.2 ± 0.6 log lux, 0.5 ± 0.5 log lux, and 0.4 ± 0.6 log lux, respectively. Light sensitive subjects manifested significantly lower VPT compared to healthy subjects. Longitudinal analysis revealed that the OPA reliably measured VPT in healthy subjects.

Noninvasive Electrical Stimulation for the Treatment of Chronic Ocular Pain and Photophobia

INTRODUCTION

"Dry eye" or "keratoconjunctivitis sicca" is a multifactorial disease estimated to have a worldwide prevalence of 5-33%. Conventional therapies targeting the ocular surface with artificial tears, anti-inflammatories, punctal closure, eyelid hygiene, and antibiotics do not provide relief in all patients, especially those with neuropathic-like ocular complaints (wind hyperalgesia and photophobia). We anticipated that ocular transcutaneous electrical nerve stimulation (TENS) would alleviate symptoms of ocular pain, photophobia, and dryness in these latter individuals.

METHODS

All individuals who received electrical stimulation between May 10, 2016 and April 6, 2017 for the treatment of chronic ocular pain at the oculofacial pain clinic of the Miami Veterans Administration Hospital were included in this retrospective review. All patients had symptoms of dryness along with other neuropathic-like symptoms (e.g., photophobia) and minimal signs of tear dysfunction. Ocular pain intensity, symptoms of dryness, and light sensitivity were compared pre-treatment and five min post-treatment via a two-tailed paired Student's t-test.

RESULTS

The use of TENS significantly reduced the mean pain intensity in both the right and left eyes five min after treatment compared to prior to treatment (p < 0.05, paired t-test). The use of TENS significantly decreased light sensitivity in both eyes (p < 0.05). The findings for symptoms of dryness, however, were equivocal with a significant decrease in the left eye but not the right (p < 0.05, paired t-test).

DISCUSSION

Our data indicate that TENS may similarly provide analgesia in patients with dry eye symptoms as it does for many other chronic pain conditions. Furthermore, the noted effect on symptoms of photophobia and dryness suggest that all may be linked by similar trigeminal-thalamic-cortical pathways. Prospective studies with electrical stimulation of dry eye are needed to further elucidate its benefit and mechanism of action.